1. Field
The following description relates to a circuit for compensating a bipolar junction transistor (BJT) base current. The following description also relates to a circuit for compensating a BJT base current, capable of decreasing a current flowed to a measuring resistor by a value between a BJT emitter current and the BJT base current, to cause a light-emitting diode (LED) driving apparatus to control a BJT constant current without an error.
2. Description of Related Art
An LED has characteristics that a driving current and a brightness of the LED may change according to a change of a forwarded voltage. However, a brightness change of the LED according to a voltage change may degrade a lighting quality when the LED is used as a lighting device.
An LED driving apparatus may constantly control a current flowing into the LED to decrease the brightness change of the LED.
FIG. 1 is a circuit diagram illustrating an LED driving apparatus 100.
Referring to FIG. 1, an LED driving apparatus 100 controls an LED current by using a BJT switch.
Further, in this example, the LED driving apparatus 100 measures a voltage VFB of the BJT emitter terminal and the LED current ILED to control the BJT based on the measured voltage VFB and the LED current ILED.
A current IR flowed to a resistor R may be represented as the LED current ILED according to mathematical equations 1 and 2 when the BJT has a common current gain β.
                              I          B                =                                            I              C                        β                    =                                    I              LED                        β                                              Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        1            
In Mathematical Equation 1, the common current gain β corresponds to any value from 50 to 200 according to an example and an IC corresponds to a current flowing into a BJT collector.
                              I          R                =                                            I              LED                        +                          I              B                                =                                                    I                LED                            +                                                I                  LED                                β                                      =                                          I                LED                            *                              (                                  1                  +                                      1                    β                                                  )                                                                        Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        2            
In Mathematical Equation 2, an IB corresponds to a current flowing into a BJT base.
For example, the common current gain β may be assumed to be 100. Although the example LED driving circuit is designed to maintain the LED current ILED as 100 mA, an actual LED current corresponds to 99 mA according to a following Mathematical Equation 3.
                              I          LED                =                                            I              R                                      1              +                              1                β                                              =                                                    100                ⁢                                                                  ⁢                mA                            1.01                        =                          99              ⁢                                                          ⁢              mA                                                          Mathematical        ⁢                                  ⁢        Equation        ⁢                                  ⁢        3            
That is, when a current IR flowing into a measuring resistor is set to control the LED current ILED, the conventional LED driving apparatus 100 has a problem that an error occurs having a ratio of “1+(1/β)”. In certain examples, such an error occurs as above on an actual LED current. In other examples, the current is controlled using a metal-oxide-semiconductor field-effect transistor (MOSFET) in lieu of a BJT. In such examples, an actual LED current is substantially the same, such that Idrain=Isource, but a cost of the MOSFET is more expensive than that of the BJT.
Certain other approaches either fail to manage the actual LED current as desired and/or require the use of a MOSFET.